FACTORING WITH CONTINUED FRACTIONS, THE PELL EQUATION, AND WEIGHTED MEDIANTS

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1 Fizikos ir matematikos fakulteto Seminaro darbai, iauliu universitetas, 6, 2003, FACTORING WITH CONTINUED FRACTIONS, THE PELL EQUATION, AND WEIGHTED MEDIANTS Jörn STEUDING, Rasa LEšEVIƒIEN E Johann Wolfgang Goethe-Universität Frankfurt, Robert-Mayer-Str. 10, Frankfurt, Germany; steuding@math.uni-frankfurt.de iauliai University, Vytauto 84, 5400 iauliai, Lithuania; rasa.slezeviciene@fm.su.lt Abstract. We investigate the Continued fraction method CFRAC) for factoring large integers N. This method is based on the arithmetic properties of the convergents to N. Using the theory of the Pell equation, we construct an innite family of explicit examples of composite numbers for which CFRAC fails. We present a new variant of CFRAC, based on weighted mediants of the convergents to N, to overcome this problem. Finally, we give an examples of a 45-digit number for which our strategy succeeds. Key words and phrases: continued fraction, factoring large integers, Pell equation, weighted mediants. Mathematics Subject Classication: 11A55, 11D Introduction It is easy to multiply integers but, conversely, it is rather dicult to nd the prime factorization of a given large integer. This is the basis of many cryptosystems in practice. It is conjectured that factoring is an NP-problem, i.e., roughly speaking, there does not exist a fast factoring algorithm. One of the rst modern factorization methods is the Continued fraction method CFRAC due to Lehmer and Powers [5]. The rst implemention was realized by Brillhart and Morrison [2] with which they factored at 13 September 1970 the 38-digit seventh Fermat number F 7 := = ;

2 J. Steuding, R. leºevi iene 121 for the current knowledge on Fermat numbers we refer to net/fermat.html#prime. Soon after CFRAC became the main factoring algorithm in practice; actually it was the rst algorithm of expected subexponential running time. Until the 1980s it was the method of choice for factoring large integers but it has a limit at around 50 digits. CFRAC relies on an old idea of Fermat and Legendre, respectively. Suppose that we are interested in the prime factorization of a large integer N. If there are integers X, Y for which X 2 Y 2 mod N and X ±Y mod N, then the greatest common divisor gcdn, X + Y) is a non-trivial factor of N. This follows immediately from the identity X 2 Y 2 = X Y)X + Y). To look randomly for pairs of squares which satisfy these conditions is hopeless. In 1929 Kraitchik proposed to search randomly for suciently many squares which lie in the same residue class mod N, such that certain combinations among them lead to non-trivial divisors of N. More precisely, having suciently many congruences x 2 j 1) ε 0j l ε 1j 1 lε 2j 2... l ε mj m mod N, where the l k are small prime numbers and the ε kj are the related exponents, by Gaussian elimination modulo 2 one may hope to nd a relation of the form δ j ε 0j,..., ε mj ) 0,..., 0) mod 2, 1) j n where δ j {0, 1}. Then, setting X = j n x δ j j and Y = 1) ν 0 l ν 1 1 lν l νm m, 2) where j n δ jε 0j,..., ε mj ) = 2ν 0, ν 1,..., ν m ), we get X 2 Y 2 mod N. This splits N if X ±Y mod N. The set of prime numbers l which are chosen to nd the congruences in addition with 1 is called factor basis. Kraitchik proposed to generate the squares x 2 j by the nearest integers to N. The much more powerful continued fraction algorithm works with the numerators of the convergents to N. These convergents are the best rational approximations which make them to important objects in the theory of diophantine approximations and equations. However, they are also very useful for generating small squares modulo N as we shall see below. There are composite integers that CFRAC cannot factorize. It is our aim to give

3 122 Factoring with continued fractions an innite family of examples for such failures and to present a renement to overcome this problem. Before we present CFRAC we recall some basic facts from the theory of continued fractions. 2. Continued fractions The powerful tool of continued fractions was rst systematically studied by the dutch astronomer Huygens in the 17th century, motivated by technical problems while constructing a mechanical model of our solar system. All results given in this paragraph can be found in the classic [6]. For a 0 Z and a j N with 1 j < N and a N 1 the expression a 0 + a a a N + 1 a N denes a nite simple continued fraction. The a j are called partial denominators. For abbreviation we write [a 0, a 1, a 2,..., a N ] for the continued fraction above. First, we shall consider [a 0,..., a N ] as a function in the variables a 0,... a N. For j N we call [a 0, a 1,..., a j ] the j-th convergent to [a 0, a 1,..., a N ] and dene p 1 = 1, p 0 = a 0 and p j = a j p j 1 + p j 2, q 1 = 0, q 0 = 1 and = a j ) The computation of the convergents is easily ruled by means of the identities and p j = [a 0, a 1,..., a j ] 4) p j 1 p j 1 = 1) j. 5) The continuous fraction expansion is not unique since [a 0, a 1, a 2,..., a N ] = [a 0, a 1, a 2,..., a N 1, 1]. By the Euclidean algorithm it is rather easy to expand a rational number into a nite continued fraction [a 0, a 1, a 2,..., a N ], which is unique if 1 < a N N.

4 J. Steuding, R. leºevi iene 123 More generally we can attach to any given real number α =: α 0 a continued fraction by the iteration α j = α j + 1 α j+1 for j = 0, 1,.... We put a j = α j, where α j denotes the greatest integer α j. Obviously, if α is rational, the iteration stops after nitely many steps, and otherwise, if α is irrational, the iteration does not stop and we get by this procedure formally) α = [a 0, a 1, a 2,...]; the right hand side is an innite continued fraction. The rst thing we have to ask whether this innite process is convergent? By 4) and 5), α p j = α j+1p j + p j 1 α j p j = 1) j α j ). 6) Since the are strictly increasing for j 2, we observe that It follows that if α is irrational, then p 0 q 0 < p 2 q 2 <... < α <... < p 3 q 3 < p 1 q 1. α p j < 1 +1 ; 7) this is Dirichlet's celebrated approximation theorem. Furthermore, the innite continued fraction exists and represents α: p j α = lim = [a 0, a 1, a 2,...]. j It is easily shown that the continued fraction expansion of any irrational number is uniquely determined. In view of 6) it becomes visible what an important role continued fractions play in the theory of Diophantine approximations. A continued fraction [a 0, a 1,...] is said to be periodic if there exists an integer l with a j+l = a j for all suciently large n. We write for short [a 0, a 1,..., a n, a j+1,..., a j+l ] = [a 0, a 1,..., a n, a j+1,..., a j+l, a j+1,..., a j+l,...]. Here and in the sequel l = lα) denotes the minimal length of a period in the continued fraction expansion of α. Lagrange's theorem gives a classication

5 124 Factoring with continued fractions of quadratic irrationals, i.e., roots of irreducible quadratic polynomials with integral coecients: an irrational number α is quadratic irrational if and only if its continued fraction expansion is eventually periodic. In particular, the partial denominators of quadratic irrationals are bounded. It can be shown that if N is not a perfect square, then [ N = N, a 1, a 2,..., a 2, a 1, 2 ] N, and all appearing a i satisfy a i < 2 N. For instance, if n is a positive integer, then n = [n, n, 2n]. 8) 3. The Continued fraction factoring method Lehmer and Powers [5] presented two slightly dierent factorization methods. One of them, the A method, is dealing with the numerators p j of the convergents of the continued fraction N whereas the other one, the P method, is working with the denominators Q j in 10) below. They proved that the only instance of the success of one method and the failure of the other is that in which the A method succeeds, the P method fails, and a factor of N appears among the P s and Q s. We shall only consider the A method. In what follows m mod N denotes the smallest residue of m modulo N in absolute value. Then CFRAC has the following form: For j = 0, 1, 2,... successively: 1. Compute the jth convergent p j of the continued fraction expansion to N. 2. Compute p 2 j mod N. After doing this for several j, look at the numbers ±p 2 j mod N which factor into a product of small primes. Dene your factor base B to consist of 1, the primes which either occur in more than one of the p 2 j mod N or which occur to an even power in just one p 2 j mod N. 3. List all of the numbers p 2 j mod N which can be expressed as a product of numbers in the factor base B. If possible, nd a subset of numbers l's of B for which the exponents ε according to the prime numbers in B sum to zero modulo two as in 1), and dene X, Y by 2). If X ±Y mod N, then gcdx + Y, N) is a non-trivial factor of N. If this is impossible, then compute more p j and p 2 j mod N, enlarging the factor base B if necessary.

6 J. Steuding, R. leºevi iene 125 Of course, to speed up the algorithm one can reduce mod N whenever it is possible. Once the number of completely factored integers exceeds the size of the factor base, we can nd a product of them which is a perfect square. With a little luck this yields a non-trivial factor of our given number by the observations from the introduction). The crucial property of the values p j is, as we shall show below, that their squares have small residues modulo N. Otherwise, CFRAC would hinge on the problem of nding an appropriate factor base B. Theorem 1. Let α > 1 be irrational. Then the convergents p j to α satisfy the inequality qj 2 α 2 p 2 j < 2α. In particular, if α = N, where N N is not a perfect square, then the residue p 2 j mod N is of modulus less than 2 N. We sketch the proof since it is essential for the running time of CFRAC. Proof. In view of 7) Thus, qj 2 α 2 p 2 j = qj 2 α p j α + p j < q2 j +1 q 2 j α 2 p 2 j 2α < 2α αq 2 j+1 ) 2α + 1 ). +1 < 2α 1 + q ) j + 1, +1 which is less or equal to zero, and proves the rst assertion of the theorem. The claim on p 2 j mod N is an immediate consequence. Therefore, the sequence of the numerators of the convergents of N provides a sequence of p j 's whose squares have small residues mod N. If the squares are generated by the nearest integers to N as proposed by Kraitchik, one observes that x 2 N grows fairly quickly. More precisely, it is approximately equal to 2 N x N, which reduces the probability that x 2 N splits completely using only primes from the factor basis. The so-called Quadratic sieve overcomes this diculty by a sifting process as in the Sieve of Eratosthenes). It is a well-known fact that CFRAC does not work for prime powers N = p k with k 2. This causes no diculties. It is quite easy to check whether a given N is a prime power or not. However, there are other examples for which CFRAC does not work. For concrete examples we study a certain Diophantine equation.

7 126 Factoring with continued fractions 4. The Pell equation The Pell equation is given by X 2 NY 2 = 1, 9) where N is a positive integer. It should be noted that Pell was an English mathematician who lived in the seventeenth century but he had nothing to do with this equation. We are interested in integral solutions. Obviously, x = 1 and y = 0 is always a solution. By symmetry it suces to look for solutions in positive integers. If N is a perfect square, we can factor the lefthand side, and it turns out that 9) has no further solutions in integers. In the sequel we assume that N is not a perfect square. Euler observed that if x, y N is a solution of 9), then the left-hand side of 9) splits over Q N) which leads to x N y = 1 ry 2 N + x y ). In view of this excellent rational approximation to N it turns out that x y is a convergent to N. The complete solution of the Pell equation is due to Legendre and Lagrange. If we write N = [ N, a1,..., a j, α j+1 ], then there exist integers P j and Q j 1 such that α j = P j + N, 10) Q j where Q j N Pj 2 ). Taking into account the periodicity of the continued fraction expansion of N, it follows that the sequence of the P j, Q j is periodic as well. It can be shown that p 2 j 1 Nqj 1 2 = 1) j Q j. 11) Furthermore, if and only if j is a multiple of the minimal period l, then Q j = 1, and the convergent p j 1 1 corresponds to a solution of the Pell equation. Thus, all solutions of 9) are given by { pkl 1, q x k, y k ) = kl 1 ) if l is even, p 2kl 1, q 2kl 1 ) if l is odd. Note that all solutions can be found via x k + y k N = ±x1 + y 1 N) ±k, where k = 0, 1, 2,....

8 5. Explicit examples for failures J. Steuding, R. leºevi iene 127 The chances for factoring N increase when we have many squares p 2 j mod N. But with regard to 11) the sequence of the denominators of the convergents to the continued fraction of N is periodic of length 2l N). Kraitchik [4] proved that the minimal period length satises l N) 0.72 N log N, where N is any integer greater than 7; it is conjectured that log N can be replaced by log log N. However, if the period of the continued fraction expansion of N is too short, then the algorithm can only produce a small factor basis, which reduces the chances for factoring N. If for example N = n 2 + 2, then by 11) 1) j Q j = p 2 j 1 n 2 + 2)q 2 j 1 p 2 j 1 mod n 2 + 2). Alternatively, with regard to 3) and 8) we can compute directly { p j mod n 2 ±n if j is even, + 2) = ±1 if j is odd. Anyway, it follows that p 2 j mod n2 + 2) = 1 or = 2. This gives with the notation used in the CFRAC algorithm X +Y or Y mod N. CFRAC does not work for numbers N = n One strategy to overcome this problem is to replace N by some kn, where k is some suitably chosen integer, hoping that the continued fraction expansion of kn has better properties, see [1] for further details. In the following section we shall present another renement of CFRAC. 6. Weighted mediants For two distinct positive reduced fractions a b, c d we dene their mediant with positive integral weights λ, µ by aλ + cµ bλ + dµ ; for λ = µ this is the so-called mediant of a b, c d which is of special interest in the theory of Farey fractions. It is easily seen that the weighted mediant lies in between a b and d. c One can show that each rational number in the

9 128 Factoring with continued fractions interval with limits a b, c d is a mediant of the upper and lower bound for a certain weight λ, µ. In view of Dirichlet's approximation theorem 7) it makes sense to measure the order of approximation of a reduced fraction a b to a given irrational α by their distance in terms of the denominator b. If we have two excellent rational approximations a b and c d to an irrational α, then the weighted mediant of a b and c d is a good approximation if the weights are suciently small, as we will show now. Firstly, aλ + cµ α α bλ + dµ a + a aλ + cµ b α b bλ + dµ a + b µ bc ad bbλ + dµ). 12) Now let a b, c d be two convergents to an irrational α > 1. By 12) we get, similarly as in the proof of Theorem 1, bλ + dµ) 2 α 2 aλ + cµ) 2 = bλ + dµ) 2 aλ + cµ α bλ + dµ aλ + cµ α + bλ + dµ bλ + dµ) 2 aλ + cµ α bλ + dµ ) 2α + aλ + cµ α bλ + dµ α bλ + dµ) 2 a ) µ bc ad + b bbλ + dµ) 2α + α a ) µ bc ad +. b bbλ + dµ) In view of 7) bλ + dµ) 2 α 2 aλ + cµ) 2 < λ 2 db d2 + 2λµ + µ2 + µ bc ad b2 2α + 1 µ bc ad + b2 bbλ + dµ) ). λ + µ d )) b Without loss of generality we may assume that d < b. By 5) we have bc dy = 1 for two consecutive convergents a b, c d. In this case we nd bλ + dµ) 2 α 2 aλ + cµ) 2 < λ 2 + 2λµ + µ 2 + µλ + µ)) < λ 2 + 3λµ + 2µ 2 )2α + 2). Thus we have proved the following statement. 2α + 1 ) b 2 + µ bdλ + µ)

10 J. Steuding, R. leºevi iene 129 Theorem 2. Let α > 1 be irrational. If a b, c d are two consecutive convergents to α with d < b, then bλ + dµ) 2 α 2 aλ + cµ) 2 < 2λ 2 + 3λµ + 2µ 2 )α + 1) for any positive coprime integers λ, µ. In particular, if α = N, where N N is not a perfect square, then aλ + cµ) 2 mod N < 2λ 2 + 3λµ + 2µ 2 ) N + 1). Hence, the squares aλ+cµ) 2 mod N of numerators of weighted mediants to consecutive convergents with weights 1 λ, µ C, where C is any constant, are bounded by N as the ordinary convergents to N. Consequently, we can also use weighted mediants in the continued fraction factoring method; the eort for factoring the squares into an appropriate factor base is approximately the same as if one works with convergents, only. 7. A renement and an example Our idea is rather simple. If the period of the continued fraction expansion of N is too short, i.e., if we cannot factor N by the congruences coming from the squares of the numerators of the convergents to N, then one can work with weighted mediants of the convergents additionally. Thus we add to the CFRAC algorithm of the Section 3 as fourth step: if the full period did not lead to a factorization of N, compute for 1 j l N) and coprime non-negative integers λ, µ the numbers p j λ, µ) := λp j 1 + µp j mod N and return to step 2 by replacing p j mod N with p j λ, µ). We shall give an example. In the case of numbers N = n these weighted mediants are λn + µn 2 + 1) and λn2 + 1) + µ2n 3 + 3n) λ + µn λn + µ2n ) We need only the squares of the numerators modulo N which are λn 2 2λµn + µ 2 and λ 2 + 2λµn + µ 2 n 2. Varying the weights λ, µ = 0, 1,... gives a plenty of good candidates for building up an appropriate factor base note that the case λ = 1 and µ = 0 yields the old CFRAC algorithm). This algorithm was implemented on a standard personal computer. For instance, we factored N = = =

11 130 Factoring with continued fractions It seems to be a good strategy to use also weighted mediants of weigthed mediants in fact, these are mediants to convergents with larger weights). If there are small prime divisors, this algorithm is rather fast and splits quite large integers. For instance, we found for the 45-digit number N = References = = [1] H. Riesel, Prime numbers and Computer methods for factorization, Basel, Birkhäuser 1985). [2] J. Brillhart, M. A. Morrison, A method of factoring and the factorization of F 7, Math. Comp. 29, ). [3] N. Koblitz, A course in Number theory and Cryptography, Berlin, Springer, 2nd ed. 1994). [4] M. Kraitchik, Recherches sur la Théorie des Nombres, tome II, Paris, Gauthier-Villars, ). [5] D. H. Lehmer, R. E. Powers, On factoring large numbers, Bull. Amer. Math. Soc. 37, ). [6] O. Perron, Die Lehre von den Kettenbrüchen. I, Leipzig, Teubner, 3rd ed. 1954). Faktorizavimas naudojant grandinines trupmenas, Pelio lygti ir mediantes su svoriu J. Steuding, R. leºevi iene Darbe nagrinejamas grandininiu trupmenu metodas CFRAC) dideliu sveikuju skai iu N faktorizacijai. Jis remiasi N reduktu aritmetinemis savybemis. Naudojant Pelio lyg iu teorij, pateikiama sudetiniu skai iu pavyzdºiu, kuriems CFRAC metodas yra neveiksmingas tokiu skai iu yra be galo daug). Straipsnyje pristatomas naujas CFRAC variantas ²ios problemos sprendimui. Jis remiasi N reduktu mediantemis su svoriu. Be to, yra pateikiamas 45 skaitmenu skai ius, kuris faktorizuotas naudojant straipsnyje apra²yt strategij.

12 J. Steuding, R. leºevi iene 131 Rankra²tis gautas